A family of guanine nucleotide binding proteins, G-proteins, function as receptor effector couplers. Two distinct G-proteins, Gs and Gi couple stimulatory and inhibitory hormone receptors respectively to adenylate cyclase. A homologous retinal protein, transducin, couples a light receptor, rhodopsin, to cGMP phosphodiesterase. Go is a G-protein of unknown function recently discovered in brain. We have purified G-proteins, raised antibodies to them, and used these to study the structure, function, and distribution of specific G-proteins. We have also used recombinant DNA methods to isolate cDNA clones for G-protein subunits, to determine their DNA sequence and primary structure, and to study mRNA synthesis and structure.